Avian flu has killed more than twice the number of wild birds previously estimated in the UK, according to data collected by the Guardian, with numbers likely to escalate during this year’s breeding season. The highly infectious variant of H5N1 has caused Europe’s worst bird flu outbreak, resulting in the most significant and sudden loss of birds in decades. Conservationists are warning that it could mean the extinction of some seabird species, as breeding colonies have been particularly badly hit. The British Trust for Ornithology (BTO) estimates that “many more than 20,000” wild birds have died in the UK since the current outbreak began in October 2021. However, the Guardian has studied data from devolved governments and nature organisations that shows the actual figure is at least 50,000 between October 2021 until the start of April 2023. Experts say this number is still likely to be a gross underestimate as there is no national monitoring and most carcasses are never found and not counted. Already this year, there are reports that black-headed gulls, which start nesting in early April – before most other seabirds - have been badly hit across the country. According to the RSPB, 1,200 birds have died on their nature reserves in total in 2023, the vast majority of which are black-headed gulls, and there have been early reports of fatalities coming in from Europe. Conservationists at the Wildlife Trust said they had “never seen mortality this high before” at Yorkshire’s largest black-headed gull colony at North Cave Wetlands nature reserve, where 10% of the 2,000 breeding pairs of black-headed gulls have died. However, there are signs they could be over the worst of it. David Craven, regional manager at Yorkshire Wildlife Trust, said: “After an initial spread through the colony, which saw a very rapid rise to 10% mortality, we are now seeing a slowing in the numbers of dead and dying birds, which leads us to hope that this means the disease within the colony is burning itself out.” NatureScot said it had not yet received any reports of mass mortalities among wild birds this year. The Guardian figures show that more than 36,000 birds have died from avian flu in Scotland, with seabirds and waterbirds the worst hit. At least 13,000 Svalbard barnacle geese died in the winter of 2021-22 – a third of the total population. In England, 7,300 deaths were recorded in total, which is divided between two sites off the Northumberland coast – the Farne Islands (an estimated 6,000 deaths) and Coquet Island (1,300 deaths estimated), where rare birds such as arctic terns, black-headed gulls and common terns were among those most affected. Reports from Wales came from a single Royal Society for the Protection of Birds (RSPB) site – Grassholm, west of Pembrokeshire – where an estimated 5,000 northern gannets died. The UK is home to 60% of the planet’s northern gannets, with the population likely to be affected for decades as they are a long-lived species that is slow to reproduce. “This is the first collation of deaths in all species since the first signs of high mortality in autumn-winter 2021,” said Dr Helen Baker, from the Joint Nature Conservation Committee (JNCC), a public body that advises the UK government. “It shows both the full extent of spread of the disease across the UK and also that the minimum estimate of losses is higher than early figures.” The data collected by the Guardian focused on major seabird and waterbird breeding and wintering grounds, whereas other species, such as raptors, are more likely to die alone, so it is harder to know the full impact of the disease. The government publishes data on positive tests for avian flu in wild bird carcasses but does not estimate how many birds have died. The data for England and Wales shows there were 1,600 positive tests between October 2021 to March 2023. A spokesperson for the Department for Environment, Food and Rural Affairs (Defra) said: “We are tackling this outbreak – which poses a significant threat to the UK’s wild bird populations – using the very latest scientific evidence and international best practice.” Separately, Northern Ireland has counted 550 deaths during this outbreak. Prof James Pearce Higgins, director of science at the BTO, said: “These updated estimates underline the magnitude of impact that avian influenza has had on wild bird populations, particularly the UK’s internationally important seabird populations. “Ongoing surveys and monitoring at seabird colonies this summer will help us quantify these impacts further, as many infected birds are likely to have died undetected,” he added. In the UK, more than 60 species have been affected by bird flu. Claire Smith, RSPB senior policy officer, described the Guardian’s figures as an “underestimate” and said: “Sadly, we are not surprised by these figures and it reflects what we saw across our nature reserves last year and what we are seeing worldwide. “It is difficult to get absolute numbers for several reasons, including the remoteness of sites, that birds will fall off into the sea and the fact that bird flu monitoring was not well set up for wild birds last year.” She added: “The outbreak continues this spring as we are seeing positive tests and high numbers of deaths of black-headed gulls in colonies across the Midlands and eastern England. We also have concerns about many raptor species.” The National Trust, which owns the Farne Islands, is already bracing itself for thousands of deaths again this breeding season as seabirds return. Already, three new positive cases of bird flu have been detected on the islands and they have been closed to visitors. Baker said the BTO and JNCC were working towards a fuller assessment of all data for individual species. She added: “Although this estimate might be slightly higher [than the Guardian figures], it will still be a minimum and it will not be possible to assess the impact for most species until after new counts of nesting birds in seabird colonies are completed over the coming summer.” Since this latest outbreak of avian flu in 2021, 8.3 million poultry birds died or have been culled in the UK, according to Defra. In total, 20 million birds are slaughtered every week for human consumption.